1. Field of the Invention
The present invention relates to a master cylinder for actuating a hydraulic cylinder for a hydraulic brake or clutch.
2. Description of the Prior Art
As shown in FIG. 1, a conventional type of master cylinder comprises a cylinder body having a cylinder bore 3 in communication with an oil tank 2 through a relief port 7 and a supply port 8 which are formed through the wall of the cylinder body in axially spaced relation with each other, and a piston 4 slidably recieved in the cylinder bore 3 to define therein at its forward end a hydraulic chamber 6 which is in communication with an output port 5. The piston 4 has a pair of forward and rear piston portions 10, 11 axially spaced from each other and an annular groove portion 9 formed therebetween, the annular groove portion having an axial width larger than the sliding stroke of the piston 4. The annular groove portion 9 is normally in communication with the oil tank 2 through the supply port 8. The forward piston portion 10 has a plurality of through apertures 12 axially formed therethrough to provide a communication between the annular groove portion 9 and the hydraulic chamber 6. A piston cup 13 is disposed in the hydraulic chamber 6 and urged under the action of a return spring 15 in a rearward direction toward the forward piston portion 10 with a spacer 14 of a resilient sheet material interposed therebetween. The return spring 15 also serves to urge the piston 4 as a whole in the backward direction.
In FIG. 1, reference numerals 16 and 17 designate a stop ring and a dust boot, respectively.
In operation, the piston 4 is forced forward, as shown by the arrow, through an operating lever not shown to move the piston cup 13 forwardly of the relief port 7 so that working oil within the hydraulic chamber 6 is pressurized to actuate a hydraulic cylinder such as a brake connected to the output port 5. When the operating force of the operating lever is released, the piston 4 is retracted by the spring force of the return spring 15 and as a result, the outer peripheral portion of the piston cup 13 is flexed forwardly towards the hydraulic chamber 6 due to a pressure difference created between the hydraulic chamber 6 and the annular groove portion 9 to form a small clearance around the outer periphery and at the same time the spacer 14 is deformed so as to open the through apertures 12. Thus, oil stored in the oil tank 2 is introduced into the hydraulic chamber 6 through the supply port 8, the annular groove portion 9, the through apertures 12 and the aforesaid small clearance to supplement a reduced or consumed portion of working oil within the hydraulic chamber 6.
In this way, the spacer 14 is provided to prevent the piston cup 13 from being damaged as it is forced into the through apertures 12 when the pressure in the hydraulic chamber 6 increases.
As described above, in the conventional master cylinder, the through apertures 12 and the spacer 14 are inevitably required to supply working oil into the hydraulic chamber 6 and to protect the piston cup 13, but it is relatively cumbersome and time-consuming to fabricate these elements.